The present invention relates to an optical system for reading from and/or writing information on an information recording and/or reproducing medium such as an optical disk and, more particularly, to an optical head for projecting a laser beam onto an information recording and/or reproducing medium to read from and write information on it.
In an optical system which reads out from or writes information on an information recording and/or reproducing medium, a laser beam is projected onto the information recording and/or reproducing medium (to be referred to as an optical disk hereinafter) through an optical head. The laser beam reflected from the optical disk is picked up by the optical head. In order to write information on the optical disk and read it out therefrom properly, the focal point of an objective lens of the optical head must be accurately placed on the light-reflecting surface of the optical disk. In other words, the beam waist of the laser beam converged by the objective lens must be projected onto the optical disk. Various apparatuses have been developed to detect the just focusing state or in-focus state of the objective lens. However, none of them is satisfactory. For example, an apparatus which utilizes the difference between sizes of focused and defocused beam spots on the optical disk is proposed wherein different patterns of the just focused and defocused beam spots are projected onto a photodetector and are detected by the photodetector to achieve proper focusing. Further, an apparatus for focusing a laser beam, which is provided with a lens system combining a convex lens and a cylindrical lens is disclosed in U.S. Pat. No. 4,079,247 of Bricot et al. In these apparatuses, if minute recess or projections are formed on the optical disk, a diffraction pattern is formed in the beam spot pattern on the photodetector, resulting in erroneous operation. Especially, in an optical disk which has a tracking guide to increase information recording capacity, a diffraction pattern of the tracking guide is formed in the beam spot pattern on the photodetector when the beam spot is formed on the tracking guide. As a result, the apparatus may be erroneously operated.
In the apparatus described above, the defocusing state of the objective lens is detected by changes in the size of the beam spot pattern on the photodetector or in the shape of the beam spot pattern. Another apparatus is proposed which detects the defocusing state of the objective lens by the location of a beam spot pattern formed on a photodetector. In this apparatus, a laser beam for detecting the defocusing state of the objective lens is incident on the objective lens projected onto the optical disk therethrough, in addition to laser beams which are used for readout and writing of information. However, it has a drawback that the optical system becomes complex and the apparatus is manufactured at high cost, because the optical system having at least two optical paths for the laser beams is required. Other apparatuses are disclosed in Japanese Patent Disclosure Nos. 53-28405, and 53-10407, respectively. In these apparatuses, laser beams for reading out and writing information are not transmitted on an optical axis of an objective lens but are transmitted in parallel to the axis thereof. However, in these apparatuses the laser beams cannot be sufficiently converged by the objective lens, and a sufficiently small beam spot cannot be formed on the optical disk. Further, since the laser beams pass through the outer peripheral portion of the objective lens and are projected onto the optical disk, the intensity of laser beams projected onto the optical disk is decreased by eclipse.
Further, an apparatus is disclosed in Japanese Patent Disclosure No. 53-118103, in which a prism is arranged on an optical path of laser beams reflected by an optical disk. In this apparatus, the intensity of the laser beams may attenuate when they pass through the prism, and unwanted diffraction may occur.
Although the various aforementioned drawbacks are peculiar to the optical system of an optical head, the inventor hereof has revealed that the photodetector provided in the system for detecting the defocusing stateof the objective lens also involves a problem to be solved.
It is possible with the known optical head that the objective lens fails to come into the just focusing state in response to a signal from the photodetector. This may occur when the objective lens lies too close to the optical disk or too far from the optical disk. If the objective lens is located too close to, or too far from, the optical disk, the level of the signal from the photodetector is low and contains much noise. Since the defocusing state is detected from this signal, the system will perform a faulty operation, thus bringing the objective lens into contact with the optical disk or holding the objective lens in defocusing state. It is therefore necessary to determine whether or not the objective lens can be brought into just focusing state, no matter whether it is in defocusing state or an abnormal state (i.e. unfocusing state).